feat(display): limit /adults and /juniors to last N months by default

Show only the last MONTHS_TO_SHOW months (default 5) in the fee table columns
so the page fits on screen without horizontal scrolling. Reconciliation still
runs over the full month history so balances, credits, and debts are unaffected.
Set MONTHS_TO_SHOW=0 to show all months. Implemented in both Python and Go.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app.py

@@ -19,7 +19,7 @@ sys.path.append(str(scripts_dir))
 
 from config import (
     ATTENDANCE_SHEET_ID, PAYMENTS_SHEET_ID, JUNIOR_SHEET_GID,
-    BANK_ACCOUNT, CREDENTIALS_PATH,
+    BANK_ACCOUNT, CREDENTIALS_PATH, MONTHS_TO_SHOW,
 )
 from attendance import get_members_with_fees, get_junior_members_with_fees
 from match_payments import reconcile, fetch_sheet_data, fetch_exceptions
@@ -33,6 +33,12 @@ from cache_utils import get_sheet_modified_time, read_cache, write_cache, _LAST_
 from sync_fio_to_sheets import sync_to_sheets
 from infer_payments import infer_payments
 
+
+def _last_n_months(months):
+    """Return the last MONTHS_TO_SHOW months; 0 means show all."""
+    return months[-MONTHS_TO_SHOW:] if MONTHS_TO_SHOW > 0 else months
+
+
 def get_cached_data(cache_key, sheet_id, fetch_func, *args, serialize=None, deserialize=None, **kwargs):
     mod_time = get_sheet_modified_time(cache_key)
     if mod_time:
@@ -175,7 +181,7 @@ def api_adults():
     )
     result = reconcile(members, sorted_months, transactions, exceptions)
     vm = build_adults_view_model(
-        members, sorted_months, result, transactions,
+        members, _last_n_months(sorted_months), result, transactions,
         datetime.now().strftime("%Y-%m"),
         attendance_url=attendance_url, payments_url=payments_url, bank_account=BANK_ACCOUNT,
     )
@@ -199,7 +205,7 @@ def api_juniors():
     adapted_members = adapt_junior_members(junior_members)
     result = reconcile(adapted_members, sorted_months, transactions, exceptions)
     vm = build_juniors_view_model(
-        junior_members, adapted_members, sorted_months, result, transactions,
+        junior_members, adapted_members, _last_n_months(sorted_months), result, transactions,
         datetime.now().strftime("%Y-%m"),
         attendance_url=attendance_url, payments_url=payments_url, bank_account=BANK_ACCOUNT,
     )
@@ -248,7 +254,7 @@ def adults_view():
     record_step("reconcile")
 
     vm = build_adults_view_model(
-        members, sorted_months, result, transactions,
+        members, _last_n_months(sorted_months), result, transactions,
         datetime.now().strftime("%Y-%m"),
         attendance_url=attendance_url,
         payments_url=payments_url,
@@ -284,7 +290,7 @@ def juniors_view():
     record_step("reconcile")
 
     vm = build_juniors_view_model(
-        junior_members, adapted_members, sorted_months, result, transactions,
+        junior_members, adapted_members, _last_n_months(sorted_months), result, transactions,
         datetime.now().strftime("%Y-%m"),
         attendance_url=attendance_url,
         payments_url=payments_url,